1 Revision 1 2 Transjordanite, Ni2P, a new terrestrial and meteoritic phosphide, and natural solid solutions 3 barringerite-transjordanite (hexagonal Fe2P–Ni2P) 4 Sergey N. Britvin1,2*, Michail N. Murashko1, Ye. Vapnik3, Yury S. Polekhovsky1†, 5 Sergey V. Krivovichev2,1, Maria G. Krzhizhanovskaya1, Oleg S. Vereshchagin1, Vladimir V. 6 Shilovskikh1,4, and Natalia S. Vlasenko1 7 1St. Petersburg State University, Universitetskaya Nab. 7/9, 199034 St. Petersburg, Russia 8 2Kola Science Center, Russian Academy of Sciences, Fersman Str. 14, 184200 Apatity, Russia 9 3Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev, 10 P.O.B. 653, Beer-Sheva 84105, Israel 11 4Institute of Mineralogy, Urals Branch of Russian Academy of Science, Miass 456317, Russia 12 †Deceased: September 29, 2018. 13 14 Abstract 15 This paper is a first detailed report of natural hexagonal solid solutions along the join Fe2P–Ni2P. 16 Transjordanite, Ni2P, a Ni-dominant counterpart of barringerite (a low-pressure polymorph of Fe2P), 17 is a new mineral. It was discovered in the pyrometamorphic phosphide assemblages of the Hatrurim 18 Formation (the Dead Sea area, Southern Levant) and was named for the occurrence on the 19 Transjordan Plateau, West Jordan. Later on, the mineral was confirmed in the Cambria meteorite 20 (iron ungrouped, fine octahedrite) and it likely occurs in CM2 carbonaceous chondrites (Mighei 21 group). In reflected light, transjordanite is white with a beige tint. It is non-pleochroic and weakly 22 anisotropic. Reflectance values for four COM recommended wavelengths are [Rmax/Rmin, % (λ, nm)]: 23 45.1/44.2 (470), 49.9/48.5 (546), 52.1/50.3 (589), 54.3/52.1 (650). Transjordanite is hexagonal, 24 space group P͞ 62m; unit cell parameters for the holotype specimen, (Ni1.72Fe0.27)1.99P1.02, are a 3 3 25 5.8897(3), c 3.3547(2) Å, V 100.78(1) Å , Z = 3. Dcalc = 7.30 g/cm . Crystal structure of holotype 1 26 transjordanite was solved and refined to R1 = 0.013 based on 190 independent observed [I > 2σ(I)] 27 reflections. The crystal structure represents a framework composed of two types of infinite rods 28 propagated along the c-axis: (1) edge-sharing tetrahedra [M(1)P4] and (2) edge-sharing [M(2)P5] 29 square pyramids. Determination of unit-cell parameters for 12 members of the Fe2P–Ni2P solid 30 solution demonstrates that substitution of Ni for Fe in transjordanite and vice versa in barringerite 31 does not obey Vegard’s law, indicative of preferential incorporation of minor substituent into M(1) 32 position. Terrestrial transjordanite may contain up to 3 wt. % Mo whereas meteoritic mineral bears 33 up to 0.2 wt. % S. 34 35 Keywords: transjordanite, barringerite, phosphide, Fe-Ni-P system, Fe2P, Ni2P, crystal structure, 36 phase transitions, solid solution, Vegard’s law, pyrometamorphism, meteorite, prebiotic 37 phosphorylation 38 39 Introduction 40 Phase relationships in the system Fe-Ni-P play a significant role in crystallization pathways of 41 metal-rich celestial bodies whose fragments are delivered as the iron and stony-iron meteorites 42 (Buchwald 1975; Goldstein et al. 2009). Iron-nickel phosphides are considered to be a reservoir for 43 reduced phosphorus in deep planetary interiors (Dera et al. 2008; Minin et al. 2018; He et al. 2019) 44 and a source of low-valent phosphorus in the prebiotic phosphorylation processes on early Earth 45 (Pasek et al. 2017; Gibard et al. 2019). Barringerite, (Fe,Ni)2P, was the second phosphide discovered 46 in nature (Buseck 1969), and is the next typical phosphide mineral following the schreibersite– 47 nickelphosphide series, Fe3P–Ni3P (Buchwald 1975; Britvin et al. 1999). Extraterrestrial barringerite 48 was described from several meteorite groups, including pallasites (Buseck 1969; Zucchini et al. 49 2018), irons (Anthony et al. 1995), howardites (Nazarov et al. 1994, Gounelle et al. 2003), 50 carbonaceous chondrites (Mikouchi et al. 2006; Nazarov et al. 2009), and in Lunar basaltic- 2 51 anorthositic breccia Yamato 793274 (Brandstätter et al. 1991). The occurrence of barringerite on 52 Earth is sometimes considered as a signature of extraterrestrial component (indication of impact 53 event) (Drake et al. 2018), though its finding in garnet peridotite (Yang et al. 2005) has an obvious 54 terrestrial origin whereas barringerite from burnt coal dumps was formed under anthropogenic 55 influence (Bariand et al., 1977; Pierrot et al. 1977; Yao 2003). Barringerite grains recovered from 56 heavy placer concentrates (Eremenko et al. 1974; Chen et al. 1983) are of debatable (probably 57 meteoritic) origin. Ni-dominant members of Ni2P–Fe2P series were reported from two Mighei-type 58 (CM2) carbonaceous chondrites, Boriskino and Allan Hills 83100 (Nazarov et al. 2009). 59 It is noteworthy that the compound Fe2P may crystallize in two polymorphic modifications: 60 the low-pressure (LP) hexagonal one having barringerite (LP-Fe2P) structure (Rundquist and 61 Jellinek 1959), and the high-pressure (HP, > 8 GPa) orthorhombic polymorph which adopts Co2Si 62 structure type (Sénateur et al. 1976; Dera et al. 2008). Synthetic Ni2P has a barringerite-type LP 63 structure to at least 50 GPa (Dera et al. 2009). Natural high-pressure (Fe,Ni)2P, allabogdanite, was 64 discovered in a small iron meteorite Onello (Britvin et al. 2002) and was recently confirmed in two 65 other irons belonging to the same Ni-rich ataxite group: Barbianello and Santa Catharina (Britvin et 66 al. 2019b). The discovery of allabogdanite raises a question of proper (either LP or HP) diagnostic 67 of natural, in particular meteoritic, (Fe,Ni)2P minerals, as there is only one meteoritic barringerite 68 (Buseck 1969) whose structural attribution was validated using an X-ray diffraction. 69 Barringerite is the most abundant phosphide mineral among terrestrial phosphide 70 associations recently reported in a pyrometamorphic suite of the Hatrurim Formation, in the vicinity 71 of the Dead Sea (in Israel and West Jordan) (Britvin et al. 2015, 2017b). During the ongoing 72 research of those phosphide assemblages, we found that a complete series of solid solutions between 73 barringerite-type polymorphs of Fe2P and Ni2P does exist in nature. The Ni-dominant member of the 74 series, a new mineral, was named transjordanite, for the type locality on the Transjordan Plateau in 75 West Jordan. Both the mineral and its name have been approved by the IMA Commission on New 3 76 Minerals, Nomenclature and Classification (IMA 2013–106). The holotype specimen of 77 transjordanite is deposited at the Mineralogical Museum of the Department of Mineralogy, Saint 78 Petersburg State University, St. Petersburg, Russia, catalogue number 19605. In the course of an 79 investigation of meteoritic phosphide assemblages from a collection of the Mining Museum, Saint 80 Petersburg Mining Institute (Britvin 2007), an interesting association of Fe2P–Ni2P minerals was 81 revealed in the Cambria iron meteorite (fine octahedrite). Recent re-examination had showed that 82 this assemblage contains Ni-dominant analogue of barringerite and thus comprises the first 83 confirmed extraterrestrial occurrence of transjordanite. Whereas synthetic compounds along the join 84 Fe2P–Ni2P are well studied owing to their promising magnetic properties (e.g., Zach et al. 2004; 85 Balli et al. 2007), wear resistance (Sudagar et al. 2013) and catalytic applications (Hitihami- 86 Mudiyanselage et al. 2015; Wexler et al. 2016), the majority of Fe2P–Ni2P minerals have no 87 characterization except of electron microprobe data. We herein provide a detailed report on naturally 88 occurring transjordanite and its solid solutions with barringerite, in comparison with the reported 89 data on their synthetic analogues. 90 91 Occurrence of transjordanite 92 The Hatrurim Formation, otherwise known as the Mottled Zone, has attracted significant 93 mineralogical interest during the past decades. This is the world’s largest, geologically juvenile suite 94 of pyrometamorphic rocks whose outcrops are scattered over the area ~30,000 km2 across the 95 territory of Israel, Palestinian Authority and West Jordan. The description of geological setting and 96 mineral assemblages of the Mottled Zone is given in a series of papers (Gross 1977; Burg et al. 97 1999; Vapnik et al. 2007; Geller et al. 2012; Novikov et al. 2013; Kolodny et al. 2014; Sokol et al. 98 2019). Phosphide assemblages of the Mottled Zone represent super-reduced part of its mineralogy 99 (Britvin et al. 2015). The minerals related to barringerite-transjordanite series are the most abundant 100 phosphides of the Mottled Zone, found both on Israel and Jordan shoulders of this formation. 4 101 Transjordanite from Jordan, including the holotype specimen, comes from the small phosphorite 102 quarry in West Jordan (31° 21' 52'' N, 36° 10' 55'' E) which crops out the so-called “paralavas” of the 103 Daba-Siwaqa complex. Paralavas comprise Cretaceous-Tertiary sediments (chalks and marls) 104 melted at the temperature beyond 1100 °C, yielding different types of remelted basic silicate rocks. 105 Phosphides, including minerals of transjordanite-barringerite series, are commonly confined to the 106 interstices between calcined unmelted sediments and paralavas (Britvin et al. 2015). In the Hatrurim 107 Basin (located a few km west of the southern subbasin of the Dead Sea, Israel), transjordanite was 108 found in an unusual diopside microbreccia occurred in the midstream of the Nahal Halamish 109 (Halamish wadi). 110 Cambria is a 16 kg iron meteorite found in 1818 nearby Lockport, Niagara County, New 111 York, U.S.A., 43° 12' N, 78° 49' W. It belongs to a group of fine octahedrites (10.4 wt. % Ni) and is 112 chemically classified as an ungrouped (anomalous) iron. A detailed description of the meteorite 113 along with a review of early references is given by Buchwald (1975) who emphasizes that Cambria 114 is a unique iron which apparently underwent a violent (perhaps shock-induced) event during its 115 escape from the parent body.